Preheat the oven to 350 degrees. Heat the 8 ounces of semisweet chocolate and butter in a bowl in the microwave at 30 second increments, stirring between each, until just melted - be careful not to burn the chocolate!
In another bowl, whisk together the flour, baking powder and salt.
In a mixing bowl, beat the eggs, brown sugar, and vanilla extract on high speed until light and fluffy. Reduce the speed to low; beat in the melted chocolate. Mix in the flour mixture until just combined. Stir in chocolate chunks. Dough will be soft and thin; this is normal.
Drop heaping tablespoons of dough 3 inches apart onto baking sheets lined with parchment. Bake until cookies are shiny and crackly yet soft in centers, 12 to 15 minutes. Cool on baking sheet for 10 minutes then transfer to a wire rack to cool completely.
After the cookies have cooled melt the white chocolate in the microwave - heat at 30 second increments until just melted. Pour the chocolate into a resealable bag; snip the tip off one of the corners to make a piping bag.
Squeeze the chocolate out of the bag onto the cookies in a decorative pattern then sprinkle evenly with candy canes and sprinkles. Let the cookies dry for at least one hour.